* New climate target, 2021-27 budget dominate EU leaders' summit
* Summit expected to approve another extension of Russia sanctions (Adds Greenpeace protest, Czech PM)
By Gabriela Baczynska, Christian Levaux and Jason Hovet
BRUSSELS/PRAGUE, Dec 12 (Reuters) - Poland, Hungary and the Czech Republic were threatening on Thursday to wreck the European Union's new push for climate neutrality by 2050, just a day after the bloc's executive trumpeted it as Europe's "man on the moon" moment.
The 27 EU national leaders will meet in Brussels and push to agree to put their bloc on net-zero greenhouse gas emissions by mid-century, luring the reluctant eastern member states with extra money to transform their economies.
With floods, fires and droughts wrecking lives around the world, Greenpeace climate activists climbed the Brussels building where the leaders were to meet, unfurling banners reading "Climate Emergency", firing off red flares and blaring fire alarm sirens. Some were detained by police.
The eastern countries want more money to fund a transition to a future of lower emissions, including a role for nuclear power which emits no carbon but which Germany and others aim to phase out.
"It is important to have certainty that nobody will stop us in the construction of nuclear power units," Czech Prime Minister Andrej Babis told reporters before leaving for Brussels.
"We have to have electricity for people, for firms, and heating. And that is the priority and I will see. I don't rule out anything."
A senior diplomat from one of those hesitant countries said of the latest draft summit decision: "For us, it definitely does not go far enough. Both in terms of nuclear, but also on burden-sharing and financial support."
Another EU diplomat estimated chances for an agreement at the summit - which must be unanimous - at 50/50.
The bloc's new chief executive proposed a Green Deal this week to mobilise 100 billion euros worth of investment to help economies move away from fossil fuels.
But Poland, Hungary and the Czech Republic are not yet on board, demanding that any decision spells out in more detail the scale and scope of financing available, and pushing to include funding for nuclear energy.
"There will certainly be an amount of arm-wrestling," said a second senior EU diplomat from a country more enthusiastic about the 2050 goal. "There will have to be new money... but some member states will be less than enthusiastic about the target of raising 100 billion euros."
The Czech Republic generates about half of its electricity from coal and wants to phase most of it out over the next 20 years, replacing part of it with new nuclear power plants.
Hungary relies on nuclear for about a third of its energy needs, with coal making up less than 15% of its energy mix. It wants to abolish coal by 2030 and replace it with a mix of nuclear, gas, renewables and energy imports.
Poland produces some 80% of its power from coal and discussions about introducing nuclear energy have not yet been settled, partly due to high costs.
The climate discussion feeds into another tricky debate, over the next long-term budget, where no agreement is expected after a latest proposal to cap joint spending at 1.087 trillion euros for 2021-27 was rejected by both the frugal camp and those seeking heavier outlay. (Reporting by Jonas Ekblom, Philip Blenkinsop, John Chalmers, Gabriela Baczynska, Andreas Rinke, Marine Strauss, Marcin Goclowski, Michel Rose, Francesco Guarascio, Jan Lopatka, Marton Dunai, Writing by Gabriela Baczynska, Editing by William Maclean and Peter Graff)
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